This invention relates in general to ballasting and starting circuitry for operating gaseous discharge lamps from D.C., and more particularly to such circuitry with a current-regulated output characteristic wherein series-pass switching means is alternatively switched between on and off conductive states and to A.C. and D.C. power conversion circuits for supplying direct current to a plurality of ballasting and starting circuits.
At the present time, use of A.C. power sources to power gaseous discharge tubes, especially those used in fluorescent lighting, by far exceed the use of D.C. power sources. This is not particularly surprising because A.C. power sources are usually more readily available than D.C. power sources. However, operation of fluorescent lamps from A.C. power sources has a number of disadvantages. One of these problems is that fluorescent lamps generate and radiate radio frequency interference (RFI). RFI is a form of electro-magnetic radiation, which among other things is known for interferring with the performance of communications systems, e.g. radio and television. The RFI is generated because, as the A.C. changes or reverses polarity during a portion of each cycle, the arc between the electrodes of the gaseous discharge tube extinguishes. The tube must then be restarted for current flow in the opposite direction and much of the RFI is generated when the arc between the electrodes of the gaseous discharge tube begins to restrike.
The constant polarity reversal of voltage and current in an A.C. power source also requires that heating be provided at both electrodes of the gaseous discharge tube. To condition a gaseous discharge tube for the striking of an arc between the electrodes, it is necessary to heat the cathodic electrode to facilitate electron emission. However, in an A.C. system, the electrode of the tube which is the cathodic electrode is constantly changing as the polarity of the voltage and current change. This necessitates the heating of both terminals.
Because the arc between the terminals of the gaseous discharge tube operating from an A.C. power source is constantly being extinguished and then reignited, lighting from a fluorescent lamp is not continuous. Instead, the lamp actually flickers. This flickering phenomenom is not noticeable to the unaided eye because the frequency of most A.C. power sources is somewhat above a frequency level which is perceptible. Nonetheless, recent behavorial and physiological studies have indicated that the inherent flickering has undesireable side-effects. Behavior and activity of children tending to be hyperactive are believed to be aggravated by the flickering. The flickering is also believed to hasten fatigue, which is a serious problem to medical personnel when attempting to differentiate between the various shadings of x-ray films.
The flickering phenomenom further causes stroboscopic effects when any movement is related to a harmonic of the A.C. power source frequency. This can present a safety hazard because the stroboscopic effects cause rotating machinery to appear to be either stationary or slowly rotating.
Even when operating from an A.C. power source, fluorescent lamps require circuitry to power and control the lamp because of the unusual load characteristics of gaseous discharge tubes. To achieve arcing between the electrodes of a gaseous discharge tube, the striking voltage of the tube must be exceeded. The striking voltage is often twice the voltage at which the tube will operate once striking of the arc between the terminals of the tube occurs. Circuitry must be provided to generate a voltage pulse of sufficient magnitude and duration to achieve striking. However, when striking of the arc occurs, current to the tube must then be limited. The current limiting function is often provided in A.C. circuits by a high leakage reactance transformer, which may constitute the bulk of the weight and expense in a fluorescent system. Gaseous discharge tubes are not susceptible to voltage regulation once arcing between the electrodes thereof is initiated because of the negative impedance characteristic of the tube. The tube will conduct an excessive amount of current to the point of self-destruction. Therefore the current to a fluorescent lamp must be limited and the particular load characteristic of the lamp will determine the voltage at which it operates at a regulated current level. For a given current, the operating voltage of the tube is a function of the length of the tube, its diameter, the types of gases within the tube and a number of other factors.
Some prior art efforts have been concerned with operating gaseous discharge tubes in conjunction with D.C. ballasting and starting circuits. These efforts have been generally centered around biasing a series pass semiconductor, usually a transistor, such that the current therethrough is limited to the desired current through the gaseous discharge tube. To compensate for a number of variables in such circuit design and the expected variations in the D.C. power source, a relatively large voltage is usually dropped across the series pass transistor. Hence, the series-pass transistor must dissipate a significant amount of power. This power is wasted energy and leads to a low efficiency of operation for the circuit. The power dissipation also requires the use of larger and more expensive semiconductors for the series-pass element. This further requires a heat sink to dissipate the heat from the transistor, and some instances, forced air ventilation thereof. Thus, such prior art ballasting and starting circuits have not met with much acceptance or commercial success, except in quite limited or specialized applications.